The hole in the ozone layer could grow significantly over the next few years, reigniting fears over skin cancer, cataracts and damage to vulnerable plant life.

According to scientists in Germany, changes in the sun's activity have delayed natural repairs to the layer of gas high in the stratosphere, and are about to trigger further ozone loss. They say the ozone layer, which shields the Earth from the worst of ultraviolet radiation, will not begin to recover until the end of the decade.

Martin Dameris, who led the research at the Institute for Atmospheric Physics in Wessling, said: "The ozone hole will stay around for another four to five years. We can't expect it to start to recover until 2010 and then it will take another 40 to 50 years to repair completely."

Ozone depletion is a largely forgotten problem since the Montreal protocol successfully reduced levels of CFC chemicals in the atmosphere, after British scientists in Antarctica reported they were destroying ozone. But some experts have been puzzled by the layer's slow recovery.

The German team pins the blame on the 11-year solar cycle, which makes the amount of solar radiation striking Earth periodically rise and fall. Scientists already knew the cycle influenced ozone, but Dr Dameris says its role in controlling the layer's recovery has been overlooked.

Reporting their findings in the journal Geophysical Research Letters, the team says: "We do not expect recovery, but a return to stronger ozone depletion and deeper ozone holes in the next few years in line with the expected lower solar activity." The sun's activity is expected to fall until the "solar minimum" in 2007/08.

An observed increase in ozone between 1997 and 2003 is partly explained by a surge in solar activity from 1997 to 2001, they say: "We do not believe a sustained reversal of ozone depletion started in the late 1990s. A recovery is only pretended."

They reached their conclusions using a computer model that looked at how the sun's activity and material spewed into the atmosphere by volcanic eruptions affected ozone. They say the computer correctly predicted levels of ozone from 1960 to 2003, making them confident it can accurately forecast what will happen next.

Ann Webb, an ozone scientist at Manchester University, said it would take decades for the ozone layer to recover fully because the banned CFCs degrade slowly in the atmosphere.

Some chemicals introduced to replace CFCs still damage ozone, she said, and there are worrying signs that climate change may be making the situation worse. Scientists are particularly worried that increased amounts of heat-trapping greenhouse gases in the lower atmosphere could start to cool the stratosphere, accelerating ozone loss.

Last spring ozone cover over the UK neared record lows following the coldest Arctic winter on record.